Light and smooth coating for paper or board, or a paint coating, formed using a composite structure

ABSTRACT

The present invention relates to a coating composition that comprises a composite structure with a body of polysaccharide, into which carbonate has been precipitated. The invention also relates to a coated paper or board product, onto which the said composition has been spread on one or both sides into one or more layers; as well as to paint that is formed of the coating composition according to the invention, or that contains the said composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in Finnish PatentApplication No. 20125746 filed Jun. 28, 2012.

TECHNICAL FIELD

The present invention relates to a coating that employs a compositestructure, the body of which consists of polysaccharide, into whichcarbonate is precipitated. In particular, the invention relates to theuse of such a carbonate polysaccharide structure in the coating layer ofpaper or board, in the coating of paints, or in materials similar topaints (for example in varnishes). Said structure improves thesmoothness, thickness, mottling, strength, opacity, and brightness ofthe coating layer of the finished paper or board or the paint coating,as well as the optical density of the ink, and the stiffness and sizingability of the coated paper or board.

BACKGROUND

The most common coating pigments, which are used in the manufacture ofpaper and board, are kaolins, talcs, gypsum, ground calcium carbonate(GCC), and precipitated calcium carbonate (PCC). Additionally, there aremore expensive special pigments, such as precipitated aluminiumsilicates, satin white, and titanium dioxide. In the manufacture ofpaper and board, coating pigments are preferably used, as they improvethe optical properties of the paper and board and the setting ofprinting ink (the printability), and make it possible to reduce thebasis weight of the paper body stock, board or coating layer; inparticular, if the coverage of the coating is good. A good coverageappears as the lack of mottling to the human eye.

Paints employ the same coating pigments mentioned above. Titanium isused in considerably larger amounts than in the coating of paper orboard, however. Another great difference, compared with the coating ofpaper and board, is that in latex paints, in particular, latex is usedconsiderably more frequently as a binder than pigments. Regardingpaints, the purpose is to form, on the surface that is to be painted, awear-resistant film coat that is as solid as possible and has a goodopacity. Additionally, various additives that improve the formation of afilm are used in paints, softening the latex particles and helping toform a solid paint film.

In both coating applications, i.e., in the coating of paper and boardand in paints, the purpose of the binder is to attach the pigments tothe base, which is to be coated, and to the coating. In paints, a solidcoating is formed, while in the coating of paper and board, the purposeof the use of binders is to ensure that the pigments of the coating andthe actual coating remain attached to the paper or board during theprinting, in particular. In the coatings of paper and board, there is nosolid film surface, due to the smaller amount of binder. The purpose ofa porous surface, among others, is to provide a suitable settingvelocity of ink for each printing method (e.g., offset, gravureprinting, flexographic printing, inkjet).

The aim is to make the surface of the base paper or board as even aspossible, so as to obtain as good a coverage as possible, by using assmall an amount of coating as possible. For that reason, the base paperor board is, typically, first pre-calendered before the coating. In amachine calender, there are metal rollers on both sides of the paper orboard; this keeps the thickness standard, but the density varies. In asoft calender, the paper or board travels between a hard andrubber-coated roller. One advantage thereof is that the densityvariation is minor. In both types of calender, generally, one metalroller is heated. There are also supercalenders and multi-rollcalenders, which comprise more than ten rollers, between which the paperor board travels. In the pre-calendering, there are often two rollersand, in the finishing calendering (super and multi-roll calendering),more than ten rollers. In the calendering, the upper and lower surfacesof the paper or board are mechanically pressed against each other. Thefinishing calendering is carried out, among others, to provide animproved pre-coating or a printable front coat. The greatestdisadvantage of calendering is that it decreases the thickness of thecoated paper or board, i.e., it increases the density of the product.Other disadvantages include the weakening of stiffness levels, strengthproperties, and optical properties.

The poor smoothness of the coated paper or board appears as poorercoverage and a need of a larger amount of coating to fill the holes onthe surface that is to be coated. When a coated surface is printed, thepoor coverage of the coating is often seen as an uneven print quality,i.e., so-called mottling. The poor smoothness of the coating or theresulting poor coverage can also cause gloss mottling or unevenness ofthe gloss in glossy coated paper and board grades. This is also referredto as the opacity and brightness mottling of the coating. Regarding themottling of the print, among others, the following types of mottling canbe distinguished: back trap, hydrophobic, single-colour, density, anddry mottling. It is believed that the mottling is caused by thedifferences in microporosity of the coating structure or an unevendistribution of binder in the coating. The mottling of the print iscaused, when more or less ink is absorbed in the mottled area of thepaper or board than in the surrounding area.

In addition to the calendering, efforts have been made to respond to theunevenness of the rough base paper or board by adding plastic pigmentsto the slip. The purpose of the use thereof is, among others, toincrease the ability of the coating to react to the irregularities ofthe base, in order to maintain the light structure of the coated paperor board by smaller calender loads in the calendering. Generally, thelight structure is also visible as an improved opacity, brightness, andoptical density of the print. Ideally, after the calendering, thecoating would have a good coverage, i.e., as little mottling aspossible, it would be light, i.e., its thickness would be high, itssmoothness good, and the other qualities of paper or board, such asgloss, would be at the target level and the mottling would be minor. Thenip pressure of the calendering can be decreased, if some coatingprovides the desired level of smoothness easier than another. In thatcase, a lighter, stiffer, and stronger coating is obtained, which stillhas the desired level of smoothness, and we would be closer to the idealthat is mentioned above.

With paints, there is no calendering, but also in this coatingapplication, an improved coverage of the surface that is to be paintedis obtained by means of a smoother coat of paint. This means that asufficient coverage can be obtained with a smaller amount of paint,whereby expensive titanium can be saved, in particular.

The picking of the coating of paper and board, i.e., the peeling of thefibres, pigments, or the surface of the coating itself is a very harmfuldefect in quality, in the printing, in particular. In the printingmachines, the material that has peeled off the coating accumulates, onthe printing surfaces and in the dampening fluid of the first printingunit (in multi-colour printing), in particular. In printed paper orboard, this is visible as mottles, missing spots, the peeling of thecoating, and other quality problems. Most generally, the picking of thecoating surface causes the peeling of single fibres and fibre bundles;at its worst, the delamination of part of the coating can take place.The picking in the printing is caused by that, for the splitting of theink in the printing nip, the force that is perpendicular to the coatingsurface exceeds the local strength of the coating surface at the outletof the printing nip. Binders are used in an attempt to prevent thesequality problems. An example of these comprises starch.

The ability of starch to form the bonding strength of the coating iscaused by its large number of hydroxyl groups, which increase itsability to form hydrogen bonds. In plants, starch is found in small(2-150 μm) granules that are separated during the manufacturing process.All plants contain starch. Starch is commercially produced from potato,tapioca, barley, wheat, rice, and corn, however, and to some extent,from tuberous roots, leguminous plants, fruit, and hybrids.

Generally, starch is not soluble in cold water. This is due to the factthat starch polymers are in a well-organized form, bound by hydrogenbonds in the starch granules. When an aqueous starch solution is heated,the starch granules first swell, whereafter single starch polymers arereleased from each other.

Starch is often treated to make the starch cationic, the stability ofthe solution is increased and/or its rheological properties are improvedat higher dry matter contents. Being a polyol, the chemical formingproducts of starch are, generally, ethers or esters. These treatmentalternatives include hydroxy-alkylation, cationization,carboxymethylation, acetylation, thermo-mechanical treatment, enzymetreatment, hydrogen peroxide treatment, sodium hypochlorite treatment,and acid treatment. Starches that dissolve in cold water, i.e.,so-called cold-soluble starches, are also provided by treating thestarch.

Native (untreated) starch is anionic; therefore, its attachment to thefibre, during the paper and board manufacturing process, without acationic treatment is poor. At present, cationic starches are ethersthat are manufactured using an epoxy chemical that contains a quaternaryammonium group. Cationized starch is the treated starch that is used themost.

Typically, the starch polymers inside the starch granules should bereleased either by adding to hot water or cooking the aqueous starchsolution. The cold-soluble starches mentioned above constitute anexception to this. Typically, the purpose of the coating slip that isused in the coating of paper or board is to provide as high a dry mattercontent as possible. Generally, an improved water retention,runnability, and quality properties are then achieved. When starch isused, therefore, efforts are made to minimize the amount of water neededin the manufacturing process of starch.

In addition to starch, other strength-improving additives have also beenused. Guar gum is a vegetable gum that is used most frequently. Guar gumand locust bean gum are seed gums and consist of galactomannan. Karayagum has a more complex structure; it is a branched polysaccharide. Theuse of vegetable gums has been limited by their price that is higherthan starch. They have also been more difficult to treat than starch.

Cellulose and nanocellulose also belong to polysaccharides.Nanocellulose or microfibrillated cellulose can be manufactured from allmaterials that contain cellulose, such as wood. The structure ofnanocellulose is considerably smaller than that of a normal cellulosepolymer; therefore, it contains a considerably larger number of hydroxylgroups that form hydrogen bonds Nanocellulose is also called:microfibrillated cellulose, nanofibrillated cellulose, nanofibres, andmicrofibres.

Accordingly, there is a need for a lighter coating of paper or board, ora paint coating, which provides a good coverage and improves theessential quality properties. The light, smooth coating gives themanufacturer of paper or board a possibility to achieve a smooth finishthat provides high-quality printing results and reduces the requiredcoating material costs. A coating that has a good coverage can alsodecrease the basis weight of the base paper or board, to some extent.Additionally, in the pigmenting of paper and board, in particular, animproved pre-coating can be achieved, compared to the known solution.

SUMMARY

An object of the present invention is to provide a new coating pigmentthat is suitable for paper and board products and paints, improvingtheir smoothness, strength, mottling, coverage, opacity, brightness, theoptical density of the print, and their sizing ability.

The purpose of the invention, in particular, is to provide a coatingpigment that contains a carbonate polysaccharide composite in itscoating structure.

The purpose of the coating pigment in question is to provide an improvedsmoothness and thickness, while keeping the brightness and opacity ofthe paper or board product and the paint, in particular, on a goodlevel. The increase in thickness provides a lighter coating structure.

Thus, the present invention relates to a coating composition for thecoating, pigmenting, or surface sizing of the paper or board product,containing the said composite structure. This composition is suitable tobe used, among others, as the coating pigment of the applicationsmentioned above.

To be more precise, the coating composition, according to the presentinvention, containing a composite structure with a polysaccharide body,into which carbonate has been precipitated; its use as a paint; and theapplication of the said paint wherein on the surface to be painted issuch that the coating composition is mixed with optional additives andbinders, whereafter the composition is spread onto the surface in wetform with a roller or by spraying or in dry form by powder painting,whereafter the coat of paint is dried.

Correspondingly, the coated paper or board product, according to theinvention, is characterized by on one or both surfaces thereof, isspread into one or more layers, and the method of manufacturing the sameis characterized by the coating composition is spread on one or bothsides of the uncoated paper or board product into one or more layers.

The present invention is multifunctional and improves variousproperties. The composite structure, according to the invention,provides, among others, an improvement in the smoothness properties ofthe products of various applications and a decrease in the mottlingthereof, as well as an improvement in the evenness of the coating.

In the case of fibre products, their important properties, such asbrightness, opacity, and printability can also be kept on a good level,while part of the raw materials of the coating can be replaced with thecoating pigment, according to the invention, due to its lighterstructure. The light structure and smoothness that are caused by the useof the composite, according to the invention, enable a decrease in thecalender pressures in the nip, whereby the compaction of the paper orboard can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will now be described infurther details with reference to the accompanying drawings where:

FIGS. 1A and 1B show SEM images of the coatings, wherein the carbonatestarch composite, according to the invention, is applied on the surfaceof LWC base paper (FIG. 1A), and a reference coating that uses GCC (FIG.1B).

DETAILED DESCRIPTION

The present invention relates to a coating composition that comprises acomposite structure, the body of which consists of polysaccharide, onwhich carbonate is precipitated.

The invention also relates to a coated paper or board product, on one orboth surfaces of which, the said composition is spread into one or morelayers.

Another application, according to the invention, comprises paints, whichare certain kinds of coatings and, correspondingly, can consist of thecoating composition, according to the invention, or contain thecomposition.

The polysaccharide is preferably selected from starch, carboxy-methylcellulose, guar gum, pectin, citosane, dextrins, galactomannan,alginates or nanocellulose or a mixture of any of the above, morepreferably from starch or nanocellulose, most suitably from starch orcold-soluble starch.

In the case of starch, its particles can be treated before using it inthe said composite structure, so that the particle is expanded, or evenfully or partly broken. Generally, this is carried out by heating which,in the case of the present invention, can be carried out, for example,in a solution that contains carbonate ions and other states of carbonatein the prevailing pH (i.e., in a so-called carbonate ion solution).

Calcium and/or magnesium carbonate is precipitated into cold-solublestarch, nanocellulose or a mixture thereof from the carbonate ionsolution by means of low pressure, heating, mixing, the dosing of anadditive that contains alkaline, or a mixture of any of the above.

The carbonate that is included in the composite structure andprecipitated into the polysaccharide is preferably calcium or magnesiumcarbonate or a mixture thereof, whereby the carbonate ion solution alsocontains calcium or magnesium ions of a mixture thereof

Regarding their crystal structure, precipitated calcium carbonates aretypically calcite, aragonite, or vaterite. Typically, calcite is foundas flaky and cubic crystal forms. Scalenohedral, rhombohedral, acicular,spherical, and prismatic crystal forms are also possible to obtain bychanging the precipitation conditions, such as the concentration ofslaked lime and the reaction temperature.

The manufacture of the composite structure according to the invention isdescribed in Finnish patent application Serial No. 20125569, and theinvention of this application relates to the use of the compositestructure according to the above-mentioned application in the coating ofpaper or board, or in paints.

“Carbonate ion solution” refers to a carbonate system that can containcarbonate ions (CO₃ ²⁻), bicarbonate ions (HCO₃ ⁻), carbonic acid(H₂CO₃), and even carbon dioxide (CO₂), but this content and the mutualratios of these states vary according to the pH of the solution.

The carbonate system, thus, refers to the change of different carbonatestates, according to the pH. The main states are as follows:

-   -   H₂CO₃        HCO₃ ⁻        CO₃ ²⁻        At an acidic pH, soluble carbon dioxide (CO₂) and, to a minor        extent, carbonic acid (H₂CO₃), are the main states of carbonate.        In the neutral (on both sides of a pH of 7) and alkaline ranges,        bicarbonate or hydrocarbonate (HCO₃ ⁻) is the main state of        carbonate up to a pH of about 10. In a highly alkaline range        (pH>10), carbonate (CO₃ ²⁻) is the main state. The alkalinity of        carbonates refers to the content of strong acid, with which the        aqueous solution can be titrated to the end point of        phenolphthalein. At that time, all of the CO₃ ²⁻ has been        converted into the ion state HCO₃—. This takes place at a pH of        about 8.3. In the most important pH range of the paper and board        manufacturing process, the pH of 6-8, bicarbonate (HCO₃ ⁻) is        the most predominant state. Furthermore, reasonably well        dissolved carbon dioxide and some carbonic acid and colloidal        carbonate have come to this pH range from the acidic range of        the pH range. The solution in the acidic pH range is herein        called the acidic carbonate ion solution.

By increasing the pH or temperature, the bicarbonate ions can be madechange into carbonate particles from the carbonate ion solution.Correspondingly, when the temperature is raised, carbon dioxide isreleased and the bicarbonate reacts with free calcium or magnesium ions,according to the following reaction equation:

Ca²⁺+2HCO₃ ⁻→CaCO3↓+CO2↑H₂O↑.

The reaction above also works, when a sufficiently warm carbonate ionsolution is subjected to low pressure or centrifugal force, so that thecarbon dioxide is released from the solution in the air.

When the pH is increased with an alkali, for example with NaOH orCa(OH)₂, the carbonate particles can be precipitated according to thefollowing reaction equations:

Ca²⁺+2HCO₃ ⁻+2NaOH→CaCO3↓+Na₂CO₃+2H₂O.

Ca²⁺+2HCO₃ ⁻Ca(OH)₂→2CaCO3↓+2OH⁻.

The bicarbonate in the acidic carbonate ion solution is assumed to helpthe generated carbonate filler to attach to the polysaccharide polymer.The bicarbonate ion, as such, includes a possibility for the formationof hydrogen bonds, which is important for the attachment ofpolysaccharides to the fibre, among others. It could be considered thatbicarbonate controls the attachment to the hydroxyl groups ofpolysaccharides before precipitating into carbonate.

When a polysaccharide that is treated with cationization is used, in theinvention, it is probable that the cationic groups help the generatedcarbonate polysaccharide structure attach to the other components of thecoating. Due to these reasons, the composite pigment is capable ofenhancing the strength bonds of the binders of the coating to thepigments that are used.

As mentioned above, for example, starch, carboxy-methyl cellulose, guargum or nanocellulose or a mixture of any of the above, more preferablystarch or nanocellulose, most suitably starch can be used as thepolysaccharide.

Amphoteric starches function in a wider pH range than cationic starches.Hence, they are advantageous polysaccharides to be used in the presentinvention. Other forming alternatives of starch includehydroxy-alkylation, cationization, carboxymethylation, acetylation,thermo-mechanical treatment, enzyme treatment, hydrogen peroxidetreatment, sodium hypochlorite treatment, and acid treatment.

Vegetable gums are more difficult to treat than starch. Correspondingly,polyacrylamide (PAM), nanocellulose, and carboxy-methylcellulose (CMC)do not need to be treated, as PAM is already found in either a cationicor anionic form, and the others already contain a sufficient number ofhydroxyl groups that form the hydrogen bonds. Consequently, in theiruntreated form, they are well suited to be used as polysaccharides inthe invention.

The applications of the above mentioned composite structure according tothe invention can include the coating of paper, coating of board, andpaints. Generally, the composite structure then functions as a coatingpigment in the end product.

In paper or board coatings or paint coatings, the composite structurecan be used together with other coating pigments or as a single coatingpigment.

The composite structure is most suitably dosed into the paint or coatingslip in as high a dry matter content as possible.

The fibre product to be manufactured can be coated, surface-sized orpigmented printing paper, packing board, kraft paper or another papergrade that uses mechanical pulp or chemical pulp fibre or both.

The Finnish patent application Serial No. 20125569 shows that, byraising the pH of the carbonate ion solution, which is at an acidic pH,to the neutral range, when the polysaccharide is in the acidic carbonateion solution (pH>6.5), by means of heating, the dosing of an additivethat contains alkaline additives, low pressure, mixing or a combinationof any or all of the above, the carbonate can be precipitated on thepolysaccharide. Additionally, it is possible to use any known additives,such as acids that influence the crystal form and structure of theprecipitated carbonate. The method of precipitating carbonate onpolysaccharide, which is described in the said patent application and,particularly, its claims, can thus also be implemented in connectionwith the formation of the coating composition of the present invention.In the manufacture of paper or board, in addition to the opacity,brightness, and printability (an improvement in the absorptionproperties of ink), this composite structure of precipitated carbonateand polysaccharide gives the surface-sized, pigmented or coated endproduct or the latex paints a decrease in mottling and an improvement ofsmoothness, above all.

Typically, the carbonate polysaccharide composite, according to theinvention, is used in the coating, surface sizing or pigmenting of paperor board, and in paints, according to the following alternatives (A-D).The coating, surface sizing or pigmenting can be carried out on one orboth sides, one or more times, alternatively, using one or moreon-machine or off-machine coating units. When the coating is carried outseveral times, drying can be carried out between the coating layers, ora new coating layer can be applied directly on top of the wet coating.The coating layer herein refers to a coating layer that is provided onthe base paper or board by coating, pigmenting or surface sizing. Thebase paper or board can be precalendered or uncalendered.

A) The composite according to the invention is mixed with a coating sliptogether with other pigments, binders, and other additives that are usedin the slip, so that the dry matter content of the slip is at least 55%.The paper or board is coated with this slip at least once. Thereafter,the dried and coated paper or board can be calendered to a desiredtarget smoothness and thickness. Other possible layers of coating orpigmenting do not necessarily contain the composite, according to theinvention, in the slip. When the paper or board is coated with more thanone layers of coating, the previous coating layer can be dried and/orcalendered before the new coating layer is spread on top of the previousone. Alternatively, the new coating layer can also be spread directly ontop of the previous layer without drying and/or calendering.

B) The composite according to the invention is mixed with a pigmentingpaste together with other pigments, binders, and other additives, whichare used in the paste, so that the dry matter content of the paste is atleast 20%. The paper or board is pigmented with this paste at leastonce. Thereafter, the dried and pigmented paper or board can becalendered to a desired target smoothness and thickness. The base paperor board, which is pigmented one or more times, can be coated with oneor more coating layers, according to alternative A, or with one or morecoating layers, which do not contain the said composite. When the paperor board is coated with more than one layer of coating, the previouscoating layer can be dried and/or calendered before a new coating layeris spread on top of the previous layer. Alternatively, the new coatinglayer can also be spread directly on top of the previous layer withoutdrying and/or calendering.

C) In surface sizing, the composite according to the invention is used,whereby the picking problem of the surface can be kept under control,and a coating is obtained, which improves the brightness, opacity, andthe optical density of the print. When the composite according to theinvention is used in surface sizing, other pigments, binders, andadditives can possibly also be used in the surface-sizing paste. Thesurface sizing can be used as the first coating on the base paper orboard before the pigmenting of alternative B and/or the coating ofalternative A, or before the spreading of pigmenting or coating layersthat contain no composite. When the paper or board is coated with morethan one layer of coating, the previous coating layer can be driedand/or calendered before the new coating layer is spread on top of theprevious one. Alternatively, the new coating layer can also be spreaddirectly on top of the previous layer without drying and/or calendering.

Consequently, one or more layers of pigmenting, or one or more layers ofcoating, or one or more layers of surface size contain the compositeaccording to the invention, whereas the other layers can either containthe said composite or not.

D) The use of the composite according to the invention in paints;particularly, to reduce the required amount of paint, due to an improvedcoverage, without weakening the picking strength, opacity or wearresistance of the paint layer.

The other additives mentioned above include other pigments, binders, andadditives, such as latexes, polyvinyl alcohol, CMC, starches, proteins,caseins, or dispersing agents, anti-foam agents, foam removers, pHregulators, hardening agents, regulators of water retention andrheology, synthetic thickeners, lubricants, optical clarificationagents, colouring agents, and microbicides.

Latex is a binder that is used the most frequently both in the paper orboard applications and paint applications. Typically, latexes arecombinations of styrene butadiene, polyvinyl acetate, and acrylatelatexes or the combinations of their different copolymers. In both ofthe applications above, efforts have been made to reduce the portion oflatex in the coating slip or paint to decrease the costs.

The amount of latex varies according to the printing method. In offsetprinting, the surface of the paper is moistened by water, whereby thepicking strength (wet strength, in particular) is of importance and,generally, 10-15 parts of latex are needed. In gravure printing, nodampening solution is used, but the smoothness of the paper is animportant property. In that case, the amount of latex is generally 4-5parts of latex.

The most common of the other binders are starch, carboxy-methylcellulose (CMC), polyvinyl alcohol, protein, and casein. Polyvinylalcohol and CMC yield the best bonding strength, but being moreexpensive than latex, they are not used to a great extent.

Regarding the other additives that are used in coating slips includedispersing agents, anti-foam agents, foam removers, pH regulators,hardening agents, regulators of water retention and rheology, syntheticthickeners, lubricants, optical clarification agents, colouring agents,and microbicides.

The base paper or board that is coated with the composite structurementioned above, particularly, contains chemical pulp fibre and/or woodfibre and/or synthetic fibre. In the present invention, the fibres canbe any natural fibres and/or synthetic fibres. In particular, the fibrescan comprise chemical pulp or mechanical pulp or a mixture thereof. Forexample, sulphate and sulphite cellulose fibres, dissolving pulp,nanocellulose, chemi-mechanical (CTMP), thermo-mechanical (TMP) pressuregroundwood (PWG), ground pulp, recycled fibre or the fibres of de-inkedpulp, can comprise the solid matter. Typically, sulphate and sulphitecelluloses are called chemical pulps, and thermo-mechanical pulp,pressure groundwood, and groundwood pulp are called mechanical pulps.

In addition to the coated, surface-sized or pigmented printing papersand packing materials, different special paper grades (e.g., wall paperbase and kraft paper), and different paints constitute goodapplications, since in these products, the smoothness and mottling ofthe coating layer are of great importance. In these applications,particularly, the surface sizing and pigmenting are essential.

The present invention also relates to a method of manufacturing thecoated paper or board product, wherein the coating composition mentionedabove is spread on one or both sides of an uncoated paper or boardproduct to form one or more layers.

The method of coating, i.e., the method of spreading the composition,can employ methods that contact the paper or board during the coating,or non-contacting coating methods. The most important non-contactingmethods are the curtain coating and spray coating. Blade coating, rodcoating, air brush coating and film transfer coating are the mostimportant contact methods of coating. The blade coating can be furtherdivided into long dwell time, short dwell time, and jet application. Inthese methods mentioned above, the coating slip is prepared in anaqueous solution, which is dried after the coating. In the coating ofpaper and board, the drying is, generally, carried out with hot air orinfrared lamps. Generally, the paints dry by air-drying. There are alsowaterless or powder coating methods, which are meant to make charged drypigments attach to the surface to be coated. These methods may alsoemploy the use of binders. The coating can be applied directly on thesurface of the manufactured paper or board without a separateintermediary rolling of ink (on-machine coating) or after anintermediary rolling of ink at a separate coating head (off-machinecoating). Both sides of the paper or board can be coated simultaneouslyor at different stages. The coating can be carried out one or moretimes, whereby drying can be carried out between the different runs ofcoating. Typically, the dry matter content of the coating slips is about60% or more, and the amount of coating is 10-30 g/m²/side. In addition,surface sizing and pigmenting are carried out, wherein the compositionof the pastes is simpler, and the dry matter contents and desiredamounts of coat are lower.

In the surface sizing, the picking strength and stiffness of the paperor board is enhanced, and any partly loose fibres and other componentscan be bound to the surface.

This also improves the smoothness, porosity, gloss, and the absorptionof ink. The surface sizing can also constitute a pre-treatment for thepaper and board that is to be coated. Starch, as such, can be used inthe surface sizing pastes in a dry matter content of about 10%. In thesurface sizing, the amount of surface size on the surface of the basepaper or board is, generally, 0.5 . . . 3 g/m²/side. A pond size pressor sym-sizer film coater is conventionally used in the surface sizing.

A light coating, i.e., pigmenting, improves the properties andprintability of the surface of the paper/board. The paste often containsa pigment that is added to the starch solution, often calcium carbonate,and possible additives. The dry matter contents in the pigmenting areabout 20-55%. The amount of coating is 1 . . . 10 g/m²/side, dependingon the paper and board grade and the purpose of use.

Regarding the coverage of the coating, the blade coating provides asurface with a poorer coverage on a rough base paper or board than, forexample, the air brush coating, film transfer coating, and thenon-contacting coating methods. This is due to the incapability of theblade coating to follow the irregularities of the surface that iscoated.

The following non-limiting examples illustrate the invention and itsadvantages.

EXAMPLES Example 1

A carbonate ion solution was prepared, so that 170 g of burnt lime (CaO)was mixed with 1000 g of water at 55° C. The Ca(OH)₂— slurry that wasthus generated was allowed to react, in the mixture, with the carbondioxide that was conducted thereto, so that the final pH was 6.2. Thissolution was allowed to sediment for 48 hours, whereafter the clearsolution was separated from the sedimented ingredient. The dry mattercontent of this solution was 42%. The carbonate ion solution that wasthus prepared was used as raw material in the cooking of the coatingstarch in the tests described below.

Example 2

The carbonate ion solution that was prepared, according to the previousexample, was used as the cooking water of starch in this example. Intest point A, 20 parts of starch were cooked (95° C., 30 minutes,mixing) together with the carbonate ion solution of Example 1, so thatthe final dry matter content of the paste was 50%. The starch that wasused was potato starch (coating quality) from Chemigate Oy. In testpoint B, the same starch was used as in test point A. In test point B,starch was first cooked (95° C., 30 minutes, mixing), and ground calciumcarbonate (Hydrocarb 90, Omya) was added thereto by gently mixing, sothat the proportion of starch is 20 parts and the final dry mattercontent is 50%.

Three different rods were used to coat an uncoated copying paper of 80g/m² to three different coat weights. The coating was carried out by alaboratory coater (RK K303 multicoater), using a velocity of 6.Thereafter, the sheets were dried in a conditioning room for 48 hours,at 23° C. and a relative humidity of 50%. Thereafter, their coat weightswere verified and they were soft-calendered in the following conditions:

-   -   50° C.    -   two nips    -   pressure of 200 kN/m

The following properties were determined from the calendered samples:

-   -   ISO brightness (Minolta Spectrophotometer 3610d), ISO 2470    -   Opacity (Minolta Spectrophotometer 3610d), ISO 2471    -   Thickness (L&W Thickness tester SE51), ISO 534    -   Picking strength of paper (Wax pick test), T 459 OM 93    -   Smoothness (Parker print surface), ISO 8791-4

The results are shown in Table 1. All of the results are normalized to acoat weight of 8g/m².

TABLE 1 The results that are normalized to a coat weight of 8 g/m².Smoothness Thickness Picking PPS-10 Test point Brightness % Opacity % μmstrength (μm A 86.5 89.7 149 18 1.4 B 83.1 85.4 135 14 2.7

Table 1 shows that, when the structure of the coating can be kept light(high thickness) and the surface of the coating smooth, excellentbrightness and opacity properties are also achieved. If the coatedsamples of both of these test points had been calendered to the samelevel of smoothness, the properties of the coating according to theinvention (test point A) would even have been better, because thecalendering could have been carried out at a lower nip pressure.

Example 3

The carbonate ion solutions that were prepared, according to Example 1,were used as the cooking water of the cooking of starch in this example.The potato starch that was used in the tests was from Chemigate Oy(Raisamyl).

In test point A, 35 parts of starch were cooked (95° C., 30 minutes,mixing) together with the carbonate ion solution of Example 1, so thatthe final dry matter content of the paste was 40%. In test point B, thesame starch was used as in test point A. In test point B, starch wasfirst cooked (95° C., 30 minutes, mixing), and ground calcium carbonate(Hydrocarb 90, Omya) was added thereto by gently mixing, so that theproportion of starch is 35 parts and the final dry matter content is40%.

Three different amounts of coating were run by the CLC coating machine(CLC 6000) on an LWC base paper of 40 g/m². A blade coating unit and avelocity of 800 m/min were used in the coating. The target amounts ofcoating were within 3-9 g/m². Thereafter, the sheets were conditionedfor 48 hours at 23° C. and a relative humidity of 50%, beforemeasurements. Thereafter, the amounts of coating thereof were verifiedand they were calendered in the same conditions as in the previousExample 2. The following properties were determined from the calenderedsamples:

-   -   ISO brightness (Minolta Spectrophotometer 3610d), ISO 2470    -   Opacity (Minolta Spectrophotometer 3610d), ISO 2471    -   Thickness (L&W Thickness tester SE51), ISO 534    -   IGT pick (AIC2-5), 4 m/s, medium viscous oil, ISO 3783    -   Smoothness (Parker print surface), ISO 8791-4

The mottling was assessed as follows. The samples were printed at afour-colour Heatset-offset line frequency of 60 l/cm, using round dots.The C70 and B70 frames on the “mottling” areas on both sides of thesamples were measured, so that some paper around them was also includedin the image. Herein, the clustering method was used, wherein themottles (0.5-10 mm) are searched by the threshold method, and groupedaccording to the size and contrast. As the measuring instrument, theHewlett-Packard ScanJet 7400C desktop scanner was used, with which theversion 3.02 of the Hewlett-Packard Precision Scan Pro system softwarewas used. In the measurement, the settings of the system software wereaccording to Table 2.

TABLE 2 Settings of the Precision Scan pro software for determining themottling. Resolution 1200 dpi Exposure time “Highlights” 235 Exposuretime “Shadows” 0 Exposure time “Midtones” 2.2 Crispening None Noiseremoval None

The results are shown in Table 3. All of the results are normalized to acoat weight of 8 g/m².

TABLE 3 The results that were normalized to a coat weight of 8 g/m².Bright- IGT Test ness, Opacity, Thickness, pick, Smoothness Mottlingpoint % % μm m/s PPS-10 μm index A 73.8 89.3 79 3.3 1.7 37 B 70.3 85.765 3.1 2.5 78

Table 3 shows that, when the structure of the coating can be kept light(high thickness) and the surface of the coating smooth, the mottlingindex is also improved (a lower value). The mottling index is an indexthat emphasizes the size and contrast of the mottles by the number. Ifthe coated samples of both of these test points had been calendered tothe same level of smoothness, the properties of the coating according tothe invention (test point A) would even have been better, because thecalendering could have been carried out at a lower nip pressure.

The selection of base paper has obviously influenced the levels ofbrightness. Any other selection would also have provided improvedbrightness results.

The SEM images of FIG. 1 show that the composite of the invention (testpoint A) provides a surface that consists of small granules (FIG. 1A).The coating that contains ground calcium carbonate (test point B)obviously constitutes a denser and coarser surface (FIG. 1B) than thetest point A, according to the invention (FIG. 1A). The enlargement ofthe figures is considerably smaller than the resolution of the humaneye. When examined visually, the smaller granule provides an improvedcoverage, due to its homogeneous and small granules.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent.

What is claimed is:
 1. A coating composition for the coating,pigmenting, or surface sizing of a paper or board product, comprisingcontaining a composite structure with a polysaccharide body, into whichcarbonate has been precipitated.
 2. The coating composition according toclaim 1, wherein the polysaccharide is starch, cold-soluble starch,carboxy-methyl cellulose, guar gum or nanocellulose or a mixture of anyof the above, preferably starch or nanocellulose, most preferablycold-soluble starch, whereby the shape of the polysaccharide body ispreferably swollen or partly or fully broken.
 3. The coating compositionaccording to claim 1, wherein the precipitated carbonate is calcium ormagnesium carbonate or a combination thereof, and at least part of thecarbonate is preferably precipitated from the solution directly into thepolysaccharide body.
 4. The coating composition according to claim 3,wherein part of the carbonate is added carbonate, which is preferablycalcium carbonate, such as precipitated calcium carbonate (PCC) orground calcium carbonate (GCC) or a mixture thereof.
 5. The coatingcomposition according to claim 1, further comprising other pigments,binders, and additives, such as latex, polyvinyl alcohol, CMC, starch,kaolin, calcined kaolin, talc, titanium dioxide, composite pigments,granule pigments, protein, casein, dispersing agents, anti-foam agents,foam removers, pH regulators, hardening agents, lubricants, regulatorsof water retention and rheology, synthetic thickeners, lubricants,optical clarification agents, colouring agents, and/or microbicides. 6.The coating composition according to claim 1, wherein a coating slip ishaving a dry matter content of at least 55%.
 7. The coating compositionaccording to claim 1, wherein a pigmenting paste is having a dry mattercontent of at least 20%.
 8. The coating composition according to claim1, wherein a surface sizing paste is having a dry matter content of atleast 2%.
 9. Use of the coating composition according to claim 1 aspaint.
 10. A coated paper or board product, wherein, on one or bothsurfaces thereof, the composition according to claim 1 is spread intoone or more layers.
 11. The coated paper or board product according toclaim 10, wherein also one or more layers of other coating, pigmentingor surface sizing pastes are spread on one or both surfaces thereof. 12.The coated paper or board product according to claim 10, wherein thecoating composition, which has been spread on its surface, has formedafter the drying one or more layers of coating, pigmenting, or surfacesizing, the basis weight of which is at least 0.05 g/m².
 13. The coatedpaper or board product according to claim 10, wherein it is asurface-sized or pigmented printing paper or packing material, or aspecial paper grade, such as a wallpaper base or kraft paper.
 14. Amethod of manufacturing the coated paper or board product according toclaim 10, wherein a coating composition containing a composite structurewith a polysaccharide body, into which carbonate has been precipitated,is spread on one or both sides of the uncoated paper or board productinto one or more layers.
 15. The method according to claim 14, wherein acontacting or non-contacting coating method is used for the spreading ofthe coating composition, whereby the non-contacting methods are selectedfrom the group of curtain and spray coating, electric field-assistedcoating, and dry powder coating; and the contacting methods are selectedfrom a group of blade, rod, and air brush coating, and film transfercoating.
 16. The method according to claim 14, wherein the coatingcomposition is prepared in an aqueous solution, and after spreading ontothe paper or board product, excess water is removed by drying,preferably with hot air or infrared lamps.
 17. The method according toclaim 14, wherein the coating is made without a separate intermediaryrolling.
 18. The method according to claim 14, wherein the coating ismade at a separate coating station after intermediary rollings.
 19. Amethod of spreading paint on a surface to be painted, wherein thecoating composition according claim 1 is mixed with optional additivesand binders, whereafter the composition is spread onto the surface inwet form with a roller or by spraying or in dry form by powder painting,whereafter the coat of paint is dried.